I think that's true at lower levels or if you're comparing only to 4E.
However, increasing HP is still very much a primary part of making creatures more difficult.
I've thankfully had a good DM in my usual face-to-face group, and they're comfortable with doing a lot of their own monster design.
But, by the book, a lot of high-level challenges still tend toward being bags of HP.
There's also a lack of granularity when it comes to resistance in 5E. So, a lot of creatures effectively do have more HP. The game simultaneously says that magic items shouldn't be assumed and designs monsters using a very binary method of determining resistance (which hinges upon having or not having items).
My opinion is that I would have rather kept the 4E-style encounter design (with more creatures and more moving parts,) but went with numbers for building monsters (and the 'physics engine' of the game world) that are more similar to older editions of D&D.